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LT3973EDD-5#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3973/LT3973-3.3/LT3973-5
13
3973fb
For more information www.linear.com/LT3973
APPLICATIONS INFORMATION
about 97.5%. This leads to a minimum input voltage of
approximately:
V
IN(MIN1)
=
V
OUT
+ V
D
DC
MAX
– V
D
+ V
SW
where V
OUT
is the output voltage, V
D
is the catch diode
drop (~0.7V), V
SW
is the internal switch drop (~0.5V at
max load), and DC
MAX
is the maximum duty cycle.
The final factor affecting the minimum input voltage is the
minimum dropout voltage. When the OUT pin is tied to
V
OUT
, the LT3973 regulates the output such that it stays
more than 530mV below V
IN
. This enforced minimum
dropout voltage is due to reasons that are covered in a
later section. This places a limitation on the minimum
input voltage as follows:
V
IN(MIN2)
= V
OUT
+ V
DROPOUT(MIN)
where V
OUT
is the output voltage and V
DROPOUT(MIN)
is
the minimum dropout voltage (530mV).
Combining these factors leads to the overall minimum
input voltage:
V
IN(MIN)
= max(V
IN(MIN1)
, V
IN(MIN2)
, 4.2V)
Note that the LT3973 will begin switching at a lower input
voltage (typically 3V) but will regulate to a lower FB voltage
in this region of operation (see the Typical Performance
Characteristics section).
Maximum Input Voltage Range
The highest allowed V
IN
during normal operation (V
IN(OP-
MAX)
) is limited by minimum duty cycle and can be calcu-
lated by the following equation:
V
IN(OP-MAX)
=
V
OUT
+ V
D
f
SW
• t
ON(MIN)
– V
D
+ V
SW
where t
ON(MIN)
is the minimum switch on time.
However, the circuit will tolerate inputs up to the absolute
maximum ratings of the V
IN
and BOOST pins, regardless of
chosen switching frequency. During such transients where
V
IN
is higher than V
IN(OP-MAX)
, the switching frequency will
be reduced below the programmed frequency to prevent
damage to the part. The output voltage ripple and inductor
current ripple may also be higher than in typical operation,
however the output will still be in regulation.
Inductor Selection
For a given input and output voltage, the inductor value
and switching frequency will determine the ripple current.
The ripple current increases with higher V
IN
or V
OUT
and
decreases with higher inductance and faster switching
frequency. A good starting point for selecting the induc
-
tor value is:
L = 1.5
V
OUT
+ V
D
f
SW
where V
D
is the voltage drop of the catch diode (~0.7V),
L is in µH and f
SW
is in MHz. The inductor’s RMS current
rating must be greater than the maximum load current
and its saturation current should be about 30% higher.
For robust operation in fault conditions (start-up or short
circuit) and high input voltage (>30V), the saturation cur
-
rent should
be above 1.5A. To
keep the efficiency high,
the series resistance (DCR) should be less than 0.1Ω, and
the core material should be intended for high frequency
applications. Table 2 lists several inductor vendors.
Table 2. Inductor Vendors
VENDOR URL
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Toko www.tokoam.com
Würth Elektronik www.we-online.com
Coiltronics www.cooperet.com
Murata www.murata.com
This simple design guide will not always result in the
optimum inductor selection for a given application. As a
general rule, lower output voltages and higher switching
frequency will require smaller inductor values. If the ap
-
plication requires
less than 750mA load current, then a
lesser inductor value may be acceptable. This allows use
of a physically smaller inductor, or one with a lower DCR
resulting in higher efficiency. There are several graphs in
the Typical Performance Characteristics section of this data
LT3973/LT3973-3.3/LT3973-5
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3973fb
For more information www.linear.com/LT3973
APPLICATIONS INFORMATION
sheet that show the maximum load current as a function
of input voltage for several popular output voltages. Low
inductance may result in discontinuous mode operation,
which is acceptable but reduces maximum load current.
For details of maximum output current and discontinu
-
ous mode
operation, see Application Note
44. Finally, for
duty cycles greater than 50% (V
OUT
/V
IN
> 0.5), there is
a minimum inductance required to avoid subharmonic
oscillations. See Application Note 19.
Input Capacitor
Bypass the input of the LT3973 circuit with a ceramic capaci
-
tor of
X7R or X5R type. Y5V types have poor performance
over
temperature and applied voltage, and should not be
used. A 4.7µF ceramic capacitor is adequate to bypass the
LT3973 and will easily handle the ripple current. Note that
larger input capacitance is required when a lower switching
frequency is used (due to longer on-times). If the input
power source has high impedance, or there is significant
inductance due to long wires or cables, additional bulk
capacitance may be necessary. This can be provided with
a low performance electrolytic capacitor.
Step-down regulators draw current from the input sup
-
ply in
pulses with very fast rise and fall times. The input
capacitor
is required to reduce the resulting voltage
ripple at the LT3973 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7µF capacitor is capable of this task, but only if it is
placed close to the LT3973 (see the PCB Layout section).
A second precaution regarding the ceramic input capacitor
concerns the maximum input voltage rating of the LT3973.
A ceramic input capacitor combined with trace or cable
inductance forms a high quality (under damped) tank
circuit. If the LT3973 circuit is plugged into a live supply,
the input voltage can ring to twice its nominal value, pos
-
sibly exceeding the LT3973’s voltage rating. This situation
is easily avoided (see the Hot Plugging Safely section).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. It stores
energy in order to satisfy transient loads and stabilize the
LT3973’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
C
OUT
=
50
V
OUT
• f
SW
where f
SW
is in MHz and C
OUT
is the recommended output
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher value
capacitor if combined with a phase lead capacitor (typically
15pF) between the output and the feedback pin. A lower
value of output capacitor can be used to save space and
cost but transient performance will suffer.
The second function is that the output capacitor, along
with the inductor, filters the square wave generated by the
LT3973 to produce the DC output. In this role it determines
the output ripple, so low impedance (at the switching
frequency) is important. The output ripple decreases with
increasing output capacitance, down to approximately
1mV. See Figure 1. Note that a larger phase lead capacitor
should be used with a large output capacitor.
Figure 1. Worst-Case Output Ripple Across Full Load Range
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor or one with a higher voltage
rating may be required. Table 3 lists
several capacitor
vendors.
C
OUT
(µF)
0
0
WORST-CASE OUTPUT RIPPLE (mV)
2
6
8
10
40
80
100
16
3973 F01
4
20 60
12
14
FRONT PAGE APPLICATION
V
IN
= 24V
V
IN
= 12V
LT3973/LT3973-3.3/LT3973-5
15
3973fb
For more information www.linear.com/LT3973
APPLICATIONS INFORMATION
Figure 2. Burst Mode Operation
Figure 3. Switching Frequency in Burst Mode Operation
Table 3. Recommended Ceramic Capacitor Vendors
MANUFACTURER WEBSITE
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
Ceramic Capacitors
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can cause problems
when used with the LT3973 due to their piezoelectric nature.
When in Burst Mode operation, the LT3973’s switching
frequency depends on the load current, and at very light
loads the LT3973 can excite the ceramic capacitor at audio
frequencies, generating audible noise. Since the LT3973
operates at a lower current limit during Burst Mode op
-
eration, the noise is typically very quiet to a casual ear. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3973. As pre
-
viously mentioned
,
a ceramic input capacitor combined
with trace or cable inductance forms a high quality (under
damped) tank circuit. If the LT3973 circuit is plugged into a
live supply, the input voltage can ring to twice its nominal
value, possibly exceeding the LT3973’s rating. This situa
-
tion is
easily
avoided (see the Hot Plugging Safely section).
Low Ripple Burst Mode Operation
To enhance efficiency at light loads, the LT3973 operates
in low ripple Burst Mode operation which keeps the output
capacitor charged to the proper voltage while minimizing
the input quiescent current. During Burst Mode opera
-
tion, the LT3973 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor. Because the LT3973 delivers power to the output
with single, low current pulses, the output ripple is kept
below 10mV for a typical application. See Figure 2.
As the load current decreases towards a no load condi
-
tion, the percentage of time that the LT3973 operates in
sleep
mode increases and the average input current is
greatly reduced resulting in high efficiency even at very
low loads. Note that during Burst Mode operation, the
switching frequency will be lower than the programmed
switching frequency. See Figure 3.
At higher output loads (above 90mA for the front page
application) the LT3973 will be running at the frequency
programmed by the R
T
resistor, and will be operating in
standard PWM mode. The transition between PWM and
low ripple Burst Mode
is seamless, and will not disturb
the output voltage.
V
OUT
10mV/DIV
V
SW
5V/DIV
I
L
200mA/DIV
5µs/DIV
FRONT PAGE APPLICATION
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 15mA
f = 600kHz
3973 F02
BOOST and BD Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see the
Block Diagram) are used to generate a boost voltage that
is higher than the input voltage. In most cases a 0.47µF
capacitor will work well. Figure 4 shows two ways to
arrange the
boost circuit. The BOOST pin must be more
LOAD CURRENT (mA)
0
0
SWITCHING FREQUENCY (kHz)
100
300
400
500
300
500 600
700
700
3973 F03
200
100 200 400
600
FRONT PAGE APPLICATION
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LT3973EDD-5#TRPBF

Mfr. #:
Buy LT3973EDD-5#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 42V, 750mA, 2.5uA Iq Step-Down Converter
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